MAHA
Summary
Microangiopathic hemolytic anemia (MAHA) is a Coombs-negative hemolytic anemia caused by mechanical destruction of red blood cells as they pass through narrowed or damaged small vessels containing fibrin strands or platelet microthrombi. It is characterized by schistocytes (helmet cells) on peripheral blood smear, elevated LDH, low haptoglobin, and indirect hyperbilirubinemia. MAHA is a key finding in TTP, HUS, DIC, malignant hypertension, and HELLP syndrome.
Detail
MAHA results from intravascular shearing of erythrocytes as they traverse vessels partially occluded by fibrin/platelet thrombi (as in TTP, HUS, DIC) or structural abnormalities (malignant hypertension, mechanical heart valves, vasculitis). The mechanical trauma fragments RBCs, producing schistocytes and helmet cells visible on peripheral smear—this is the diagnostic hallmark. Because hemolysis is intravascular, labs show elevated LDH, decreased haptoglobin, increased reticulocyte count, indirect hyperbilirubinemia, and a negative direct/indirect Coombs test (distinguishing MAHA from autoimmune hemolytic anemia).
Key causes include: Thrombotic thrombocytopenic purpura (TTP) due to ADAMTS13 deficiency leading to uncleaved vWF multimers causing platelet microthrombi; Hemolytic uremic syndrome (HUS), often from Shiga toxin-producing E. coli (O157:H7) causing endothelial damage; Disseminated intravascular coagulation (DIC), triggered by sepsis, trauma, or malignancy, with widespread fibrin deposition; malignant hypertension causing fibrinoid necrosis of small vessels; HELLP syndrome (Hemolysis, Elevated Liver enzymes, Low Platelets) in pregnancy; and mechanical mechanisms from prosthetic heart valves or vasculitis.
Clinical significance: MAHA often coexists with thrombocytopenia (consumption of platelets) and can lead to end-organ damage (renal failure in HUS, neurologic symptoms in TTP). Treatment targets the underlying cause—plasma exchange for TTP, supportive care for HUS, treating the underlying trigger for DIC. Recognizing schistocytes on smear in the correct clinical context is a classic USMLE testing point to differentiate MAHA from other anemias (e.g., autoimmune hemolytic anemia, which shows spherocytes and positive Coombs test).
Sources
- First Aid for the USMLE Step 1
- Robbins and Cotran Pathologic Basis of Disease
- Harrison's Principles of Internal Medicine
- UpToDate: Approach to the patient with suspected microangiopathic hemolytic anemia
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